The present invention relates to magnetic resonance imaging (MRI), and in particular to quantitation and standardization of both medical and industrial applications of MRI.
Magnetic resonance measurements have been used in both non-medical and medical applications. In a typical non-medical application, a sample or a body of non-living matter is subjected to a static magnetic field and an oscillating radiofrequency field. The radiofrequency field electrically excites hydrogen atoms in the sample or body. After the oscillating field is turned off, the intensity of proton oscillation is measured, e.g., using an antenna, typically configured to detect the intensity of oscillations in a plurality of locations in a two-dimensional plane.
In a typical medical application of MRI, a patient is placed within the bore of a large, donut-shaped magnet. The magnet creates a static magnetic field that extends along the long (head-to-toe) axis of the patient""s body. An antenna (e.g., a coil of wire) is also positioned within the bore of the large magnet, and is used to create an oscillating radiofrequency field that selectively excites hydrogen atoms (protons) in the patient""s body into oscillation. The oscillating field is then turned off, and the antenna is used as a receiving element, to detect the proton oscillations as a function of position within the body. Typically, the intensity of the oscillations is measured throughout a two-dimensional plane. When the intensities are displayed as a function of position in this plane, the result is an image that often bears a striking resemblance to the actual anatomic features in that plane. Although MRI typically involves visual display of data, xe2x80x9cimagingxe2x80x9d can involve purely digital analysis and output so that, in this context, xe2x80x9cimagingxe2x80x9d does not necessarily require visually-perceptible output.
The intensity of proton oscillations detected at a given point in the patient""s body is proportional to the proton density at that point. Because different types of tissues have different proton densities, different tissue types usually have different image intensities, and therefore appear as distinct structures in the MR image. However, the signal intensity also depends on physical and chemical properties of the tissues being imaged. In a simplified model of MRI, the detected signal intensity, as a function of position coordinates x and y in the plane being imaged is proportional to:
(1xe2x88x92exe2x88x92TR/T1)exe2x88x92TE/T2xe2x80x83xe2x80x83(1)
The parameters TR (recovery time) and TE (echo delay time) are under the control of the operator of the MR imaging system, and are constants for any given image. However, T1 and T2 are functions of the tissue under examination, and therefore vary with position in the x-y plane. By suitable selection of parameters TR and TE, either the T1 or the T2 term in Equation 1 can be made to dominate, thereby producing so-called xe2x80x9cT1-weightedxe2x80x9d and xe2x80x9cT2-weightedxe2x80x9d images, respectively.
One of the more important medical uses to which MRI has been put to date is to noninvasively scan a portion of a patient""s body, in an attempt to identify benign or malignant tumors. When MRI is used in this fashion, it is necessary to have some methodology for concluding that a given portion of an MR image represents tumor, as opposed to other tissue types such as fat, cyst, etc. One known approach to identifying tissue type has been to acquire multiple MR images of the same region of the patient""s body, using different imaging parameters, e.g., using different value of the TR and TE parameters. To take a simplified example, if it were known that a given tumor produced a high image intensity at a first parameter setting, a low image intensity at a second parameter setting, and a high image intensity at a third parameter setting, then a portion of a patient""s body that produced that pattern of intensities (high, low, high) could be tentatively identified as tumor.
Pattern recognition approaches of this type are described in U.S. Pat. No. 5,003,979. This patent describes a system for the detection and display of lesions in breast tissue, using MRI techniques.
Many previous applications of magnetic resonance measurements have been directed to determining whether a substance or tissue is or is not of a particular type (e.g., whether a portion of a body being imaged is or is not fat). Other applications have been directed to determining whether a portion of a body being imaged falls into one of a small number of discrete categories (e.g., fat, cyst, or tumor). Non-parametric magnetic resonance imaging techniques have typically not been used for effectively and efficiently quantitizing a continuous property (such as viscosity or concentration of a substance within a body or sample).
In previous MRI techniques, analysis of results have often included a subjective or non-automatic component, such as a step of classifying or identifying portions of an image using judgment of a skilled observer. Accordingly, it would be useful to include techniques to more objectively or automatically categorize or analyze MR data.
In many cases, comparison of the pattern of intensities of a patient""s tissue to xe2x80x9cstandardxe2x80x9d patterns for different tissue types does not produce results of sufficient accuracy. One problem appears to be that attempts to define a single xe2x80x9cstandardxe2x80x9d pattern for a given tissue type does not take sufficient account of possible variability in tissue of a given type. Another problem appears to be that there is substantial variability from one patient or sample to the next as well as from one MRI machine to the next or within different regions or fields of view of the same MRI machine.
Cancer treatment often includes detecting when a primary tumor has spread to other sites in the patient""s body, to produce so-called secondary tumors, known as metastases. This process, using MRI or other imaging techniques, is often complicated by the fact that a remote lesion discovered during staging could represent either a metastasis or a benign incidental finding. A number of benign lesions (such as hepatic hemangiomas and nonfunctioning adrenal adenomas) occur as frequently in patients with a known primary tumor as they do in the general population.
Resolving this dilemma requires additional imaging studies or biopsy, but often significant uncertainty persists. Biopsy may expose the patient to substantial risk when the lesion in the brain or mediastinum, or when the patient has impaired hemostasis. Even when biopsy does not present a significant risk to the patient, it may be technically challenging, such as sampling focal lesions in vertebral marrow.
For the reasons set forth above, it would be useful to have a method that could use MR data to estimate the value of a continuous property such as concentration, viscosity or the like for both industrial applications and medical applications. It would further be useful to have a method which reduces or eliminates the need for subjective analysis of MRI data.
It would also be useful to provide a method that takes account of variability among xe2x80x9cstandardxe2x80x9d substances or tissue types, takes account of variability from patient to patient, among different MRI machines and within different regions or fields of view of the same MRI machine. Particularly with regard to medical applications, it would be useful to provide a method that could noninvasively measure the similarity between a known primary tumor and a remote lesion of unknown tissue type. The clinician would use the measured similarity to determine the likelihood that the two lesions represent the same tissue. Such a method could be used to distinguish a pathological fracture from a benign osteoporotic compression fracture in a patient with a known primary tumor. Similarly, the method could be used to distinguish a metastasis from an infarction in a patient with lung cancer who presents with a supratentorial solitary enhancing lesion. Using the computed similarity to determine the likelihood that two lesions represent the same tissue would significantly improve the confidence of noninvasive imaging diagnosis.
According to the present invention, magnetic resonance data is provided in a way which is useful for both medical and non-medical (e.g., industrial) applications. In one embodiment, an MRI apparatus is used to produce a training set comprising one or more training samples. The training set is formed from a congruent set of first images of a training region of the body being studied. The term xe2x80x9ccongruentxe2x80x9d refers to the fact that each of the first images represents the same physical slice or plane through the body. In an industrial application, the training region may be, e.g., a sample of a known substance or a sample of a substance with a particular characteristic such as, e.g., a sample of oil having a known viscosity. In a medical application, the training region may be, e.g., the region of a known primary tumor. The first images are produced using a predetermined set of MRI pulse sequences that differ from one another. Each first image comprises an array of pixels, and each training sample comprises a spatially aligned set of pixels from each first image.
MRI apparatus is also used to produce a test set comprising a plurality of test samples. The test set is formed from a congruent set of second images of a test region of the test body. In an industrial application, the test region may comprise, e.g,. an unknown substance or a substance with unknown characteristics, e.g., an oil with unknown viscosity. In a medical application, the test region may comprise, e.g., a region to be scanned for a secondary tumor. The second images are produced using the same MRI pulse sequences as the first images. Each second image comprises an array of pixels and each test sample comprises a spatially aligned set of pixels from each second image.
According to one embodiment, for each test sample, one then produces similarity data indicating the degree of similarity between the test sample and the training samples. A display is then produced based upon the similarity data. The display identifies the test samples having the highest degree of similarity to the training samples. For example, one of the second images may be displayed using a conventional gray scale, while the most similar pixels are highlighted in color. In an industrial application, the display might, e.g., highlight those samples, among a plurality of oil samples, having a viscosity matching the viscosity of a training set. In the secondary tumor example, the regions of the second image that are highlighted in color will correspond to those regions most similar to the first region (the training set) which comprises the primary tumor. The color highlighted regions will therefore identify possible sites of secondary tumors.
In another aspect, the invention also provides for the generation of spatial correlation images based on each of the first and second images, and the use of the spatial correlation images in combination with the first and second images to produce the training and test samples. Instrument standardization techniques may also be applied, to minimize errors when the first and second images are acquired from different planes through the body, or at different times. In another aspect, the present invention may provide a technique for suppressing or enhancing certain tissue types in an MR image.
According to one aspect of the invention an MRI apparatus and method useful for both industrial applications and medical applications is provided. The apparatus and procedures are capable of estimating the value of a continuous property, such as concentration, viscosity or the like by interpolating or extrapolating from a model derived from training sets of data representing measurements of samples with known properties. A number of techniques are provided for objectifying the analysis. Cluster analysis techniques can be used to supplement, assist or replace subjective judgments by trained operators. Calculations or judgments regarding similarity can be made with respect to stored libraries of signatures, particularly where the library of stored signatures is obtained objectively, e.g., using cluster analysis, standardization and calibration. The signatures can be expanded signatures which include non-MR as well as MR data. Inhomogeneities in the field of a particular MR device can be corrected for based on measurements of a reference standard having a homogeneous make up. MR measurements taken through different planes of a body or different times can be standardized by including, in at least some of the fields of view, a reference standard which has a known MR signature.
According to one embodiment, the invention includes a method for identifying which, among a first plurality of regions in a first non-homogenous part of a body are most similar to a second plurality of regions in a second non-homogeneous part of a body, comprising obtaining MR measurements of each of said first and second parts of said body, defining first and second pluralities of clusters of regions of said first and second part of said body, respectively, using cluster analysis, the regions in each cluster of regions of said first and second pluralities of clusters having similar MR measurements, forming first visual displays, each of said first visual displays, including said first part of said body, said first visual displays including visual indicia identifying at least some of said first plurality of clusters or regions, selecting one of said first plurality of clusters, based on said first visual displays, as a first cluster of interest, forming second visual displays, each of said second visual displays including said second part of said body, said second visual displays including visual indicia identifying at least some of said second plurality of clusters, selecting one of said second plurality of clusters, based on said second visual displays, as a second cluster of interest, and calculating a measure of similarity between the MR measurements for said first clusters of interest and the MR measurements for said second cluster of interest.
According to another embodiment, the invention includes a method for identifying which, among a first plurality of regions in a first non-homogenous part of a body are most similar to a second plurality of regions in a second non-homogeneous part of a body, comprising obtaining MR measurements of each of said first and second parts of said body, defining first and second pluralities of clusters of regions of said first and second part of said body, respectively, using cluster analysis, the regions in each cluster of regions of said first and second pluralities of clusters having similar MR measurements, calculating a measure of similarity between MR measurements for a plurality of pairs of clusters, each pair of clusters in said plurality of pairs of clusters comprising a cluster from said first plurality of clusters and a cluster from said second plurality of clusters, selecting at least some of said pairs of clusters, based on said measures of similarity, as pairs of interest, and forming visual displays including visual indicia distinguishably identifying at least said pairs of interest.
According to another embodiment, the invention includes a method of using magnetic resonance imaging (MRI) to produce an image of a body, the method comprising the steps of using an MRI apparatus to produce a training set comprising one or more training samples, the training set being formed from a plurality of congruent first images of a training region of the body, each first image being produced using an MRI pulse sequence different from the pulse sequences used to produce the other first images, each first image comprising an array of pixels, each training sample comprising a spatially aligned set of pixels from each first image, using an MRI apparatus to produce a test set comprising a plurality of test samples, the test set being formed from a plurality of congruent second images of a test region of the same body, the second images being produced using the same MRI pulse sequences as the first images, each second image comprising an array of pixels, each test sample comprising a spatially aligned set of pixels from each second image, producing similarity data, based on cluster analysis, indicating, for each test sample, the degree of similarity between the test sample and the training samples, and producing a display based upon the similarity data.
According to another embodiment, the invention includes a method for identifying the composition of regions of a body comprising storing a first plurality of MR measurements of a substance having a first composition, storing a second plurality of MR measurements of a substance having a second composition, obtaining MR measurements of a non-homogeneous portion of said body, identifying at least a first region of said non-homogeneous portion of said body by applying cluster analysis to said MR measurements, calculating measures of similarity between the MR measurements for said first region and at least said first and second plurality of MR measurements, and identifying one of said first composition and said second composition as the composition of said region based on said measures of similarity. In this case, the method can also include displaying at least one image of said body, with visual indicia based on composition of said region and/or displaying a plurality of images of said body in real time to provide an indication of changes or movement of said first or second composition and/or a step of standardization.
According to another embodiment, the invention includes a method for identifying the composition of regions of a body comprising obtaining MR measurements of a portion of said body, including said region, obtaining a second measurement of said portion of said body, said second measurement being different from said MR measurement, and identifying the composition of said region using both said MR measurement and said second measurement. The step of obtaining MR measurements can include recalling at least some of a library of stored MR measurements from a memory device. The step of identifying include calculating a measurement of similarity by combining said first measurement of similarity with said second measurement of similarity. Preferably, the portion of said body is a substantially non-homogeneous portion.
According to another embodiment, the invention can include a method for estimating the volume occupied by a substance within a body comprising obtaining first MR measurements of a first plurality of regions in said body, said first plurality of regions substantially defining a first surface passing through a portion of said body each of said first plurality of regions being substantially representative of a volume of said body having a dimension substantially transverse to said first surface, obtaining a second MR measurement of a second plurality of regions in said body, said second plurality of regions substantially defining a second surface, different from said first surface, each of said second plurality of regions being substantially representative of a volume of said body having a dimension substantially transverse to said second surface, identifying a plurality of target regions in said body based on said MR measurements, said target regions having MR measurements which indicate said target region comprising said substance, said target region including at least some of said first and second pluralities of regions, and calculating the sum of the volumes which said target region are representative of.
According to another embodiment, the invention can include a method of using magnetic resonance imaging (MRI) to produce an image of a body, the method comprising the steps of using an MRI apparatus to produce a training set comprising one or more training samples, the training set being formed from a plurality of congruent first images of a training region of the body, each first image being produced using an MRI pulse sequence different from the pulse sequences used to produce the other first images, each first image comprising an array of pixels, each training sample comprising a spatially aligned set of pixels from each first image, using an MRI apparatus to produce a test set comprising a plurality of test samples, the test set being formed from a plurality of congruent second images of a test region of the same body, the second images being produced using the same MRI pulse sequences as the first images, each second image comprising an array of pixels, each test sample comprising a spatially aligned set of pixels from each second image, producing similarity data indicating, for each test sample, the degree of similarity between the test sample and the training samples, and calculating a volume by identifying those pixels having at least a first degree of similarity and multiplying the number of said pixels by the volumes of said body represented by said pixels.
According to another embodiment, the invention includes a method of accounting for inhomogeneities in fields produced by an MR apparatus comprising positioning a first substantially homogeneous reference material at least within a first field of view of said MR apparatus, obtaining first MR measurements in a first plurality of region of said reference material, said first plurality of regions being within said first field of view, calculating correction factors for at least some of said first plurality of regions, based on said MR measurements of said reference material, obtaining second MR measurements in a second plurality of regions of a test substance, said second plurality of regions being within said first field of view, and combining said correction factor with said second MR measurements to provide corrected MR measurements for said second plurality of regions. The steps of calculating comprise dividing first values by the MR intensity for each of said first plurality of regions. The first value can be an average intensity. Each of said second plurality of regions can be spatially coupled with one of said first plurality of regions and the step of combining can include multiplying the intensity for each of said second plurality of regions by the corresponding correction factor. The reference material can be, e.g., a cuprous sulfate solution. The first MR measurements can comprise measurements using at least first and second sequences and the method can also include calculating separate correction factors for said first and second sequences and/or different locations within the test substance.
According to another embodiment, the invention includes a method of using magnetic resonance imaging (MRI) to produce an image of a test object, the method comprising the steps of using an MRI apparatus to produce a training set comprising one or more training samples, the training set being formed from a plurality of congruent first images of a training region of a first object, each first image being produced using an MRI pulse sequence different from the pulse sequences used to produce the other first images, each first image comprising an array of pixels, each training sample comprising a spatially aligned set of pixels from each first image, at least some of said first images including an image of at least one training set reference object, using an MRI apparatus to produce a test set comprising a plurality of test samples, the test set being formed from a plurality of congruent second images of a test region of the test object, the second images being produced using the same MRI pulse sequences as the first images, each second image comprising an array of pixels, each test sample comprising a spatially aligned set of pixels from each second image, at least some of said second images including an image of at least one test set reference object substantially similar to said test set reference object, producing similarity data indicating, for each test sample, the degree of similarity between the test sample and the training samples, correcting at least part of said similarity data based on differences between said image of said training set reference object and said image of said test set reference object, and producing a display based upon the similarity data.
According to another embodiment, the invention can include a method of using magnetic resonance imaging (MRI) to produce an image of a body, the method comprising the steps of using an MRI apparatus to produce a training set comprising one or more training samples, the training set being formed from a plurality of congruent first images of a training region of the body, each first image being produced using an MRI pulse sequence different from the pulse sequences used to produce the other first images, each first image comprising an array of pixels, each training sample comprising a spatially aligned set of pixels from each first image, using an MRI apparatus to produce a test set comprising a plurality of test samples, the test set being formed from a plurality of congruent second images of a test region of the same body, the second images being produced using the same MRI pulse sequences as the first images, each second image comprising an array of pixels, each test sample comprising a spatially aligned set of pixels from each second image, producing similarity data indicating, for each test sample, the degree of similarity between the test sample and the training samples, producing a first image based on at least some of said second congruent images, producing a second image which displays only those portions of the first image which are within a user-defined similarity threshold of a portion of said training set, and subtracting said second image from said first image to produce a third image.
According to another embodiment, the invention can include a method of using magnetic resonance imaging (MRI) to produce an image of a body, the method comprising the steps of using an MRI apparatus to produce a training set comprising one or more training samples, the training set being formed from a plurality of congruent first images of a training region of the body, each first image being produced using an MRI pulse sequence different from the pulse sequences used to produce the other first images, each first image comprising an array of pixels, each training sample comprising a spatially aligned set of pixels from each first image, using an MRI apparatus to produce a test set comprising a plurality of test samples, the test set being formed from a plurality of congruent second images of a test region of the same body, the second images being produced using the same MRI pulse sequences as the first images, each second image comprising an array of pixels, each test sample comprising a spatially aligned set of pixels from each second image, producing similarity data indicating, for each test sample, the degree of similarity between the test sample and the training samples, producing a first image based on at least some of said second congruent images, calculating, for each of a plurality of,pixels within at least a part of said first image, a difference value indicating the magnitude of the difference between the MR data corresponding to said pixel and at least a first portion of the MR data from said training set, and producing a second image including visual indicia indicating, for said plurality of pixels at least first and second different levels based on said difference value.
According to another embodiment, the invention can include a method of using magnetic resonance imaging (MRI) to produce an image of a test object, the method comprising the steps of using an MRI apparatus to produce a training set comprising one or more training samples, the training set being formed from a plurality of congruent first images of a training region of a first object, each first image being produced using an MRI pulse sequence different from the pulse sequences used to produce the other first images, each first image comprising an array of pixels, each training sample comprising a spatially aligned set of pixels from each first image, using an MRI apparatus to produce a test set comprising a plurality of test samples, the test set being formed from a plurality of congruent second images of a test region of the test object, the second images being produced using the same MRI pulse sequences as the first images, each second image comprising an array of pixels, each test sample comprising a spatially aligned set of pixels from each second image, producing similarity data representing distance in a multi-dimensional measurement space producing a display based upon the similarity data.